Binding sites for human interferon-gamma in protocerebrum and hemolymph oftobacco hornworm (Manduca sexta) larvae differ in sensitivity to polycationic peptides

We have recently characterized specific binding sites for human interferon- gamma on particulates prepared from the protocerebrum and hemolymph of toba cco hornworm larvae, Manduca sexta {(Parker, M.S., Ourth, D.D., 1999. Comp. Biochem, Physiol. B 122, 155-163). The sensitivity to sulfated polysacchar ides indicated an involvement of oligobasic epitopes of hIFN-gamma in the b inding. In the present study, we found that polycationic peptides inhibited the binding of [I-125]hIFN-gamma to particulates from either the hemolymph or the protocerebrum of Manduca sexta larvae. With amino acid homopolymers , the rank order of potency was poly-L-lysine > poly-L-arginine >> poly-L-o rnithine, while the acidic side chain polymer poly-L-aspartate was not inhi bitory. However, the potency of all polycationic peptides was at least thre e-fold greater at the hemolymph particulates. Also, acidic polysaccharides such as heparin were much more efficacious in the inhibition of hIFN-gamma binding to hemolymph relative to protocerebral particulates. The peptide po lycations inhibited the binding of [I-125](Leu(31),Pro(34))human peptide YY , a ligand selective for the Y-1 subtype of the neuropeptide Y receptor, to rabbit kidney or to parietal cortex particulates with the expected rank or der of poly-L-arginine > poly-L-lysine >> poly-L-ornithine, and with little cross-tissue difference in affinity. The selectivity observed with M. sext a particulates indicates a preferential involvement of oligobasic lysine-ri ch C-terminal sequences of IFN-gamma, while large insect tissue-related aff inity differences point to involvement of diverse oligoacidic sequences in binding to protocerebrum and hemolymph sites. This study provides evidence for the presence of molecules in lepidopteran larvae that are similar in st ructure to vertebrate co-receptors of IFN-gamma, and adds to the characteri zation of these binding sites. (C) 2000 Elsevier Science Inc. All rights re served.